This invention relates generally to control valves and more particularly to four way directional air valves of the type used to control an air motor that drives a winch or hoist although its use is not limited to these applications.
To operate at maximum efficiency, air motors require a compact four way directional control valve with a large flow capacity designed to mount directly on to the motor. Operation of the valve must be by manual means or by air pilot actuator. The valve when manually operated must be capable of turning a winch motor in either direction when the handle is pulled in the same direction. In applications where the motor causes a load to be raised, a varying amount of restriction at one or other exhaust port on the control valve may be required to slow the downward speed of the load. Because of the special requirements these valves are not available `off the shelf` so they are custom designed and manufactured in low quantities which makes them many times more costly than a standard high volume valve.
Valves currently being used for these applications fall into three design categories: linear spool, rotary spool and poppet.
Linear or rotary spool type valves are basically simple in that there is only a single machined bore in the valve housing into which a spool sleeve is fitted, the moving or rotating spool being located inside the sleeve. The disadvantages of this type of valve are that they require extremely close fits between the spool and the sleeve to reduce air leakage, making them expensive to machine, intolerant to dirt and sensitive to distortion which causes the spool to jam when mounting the valve on the motor. Even with close fits, air leakage is still considerable and the spool and sleeve are usually made of heat treated stainless steel to prevent seizing in corrosive environments which adds further to their cost. When exhaust restriction is required on linear or rotary spool valves it is usually accomplished by limiting the spool travel in one direction or by providing a special spool or spool sleeve with a small hole drilled at the appropriate exhaust location. Limiting the spool travel is not usually satisfactory since it causes the air to be restricted into the motor as well as out of it and often the motor will not turn when there is little or no descending load to assist it. Using a special spool sleeve works well except it is not adjustable and a new sleeve is required for each new application. Pilot operation of linear spool valves is generally straightforward but rotary spool valves are not easy to pilot operate since they require a rotary actuator.
Current four way poppet valve designs have two chambers machined into a housing with two poppets seated in each chamber. One poppet in each chamber connects to an air inlet passage whilst the remaining poppet in each chamber connects to an exhaust passage. The poppets which are held against their respective seats by springs are lifted off their seats by a linkage on the valve handle so that to turn the motor in a particular direction, an inlet poppet in one chamber is lifted at the same time that an exhaust poppet is lifted in the other chamber, thus allowing air to enter one side of the motor and exhaust out of the other.
Poppet type valves do not require the machined precision of the spool type valves, they are able to operate with little or no air leakage and they tolerate dirt. To hold the poppets against the maximum expected operating pressure however, heavy springs or a balancing piston on the stem of each poppet is required which, in either case, adds to the cost. Because of the double chamber and handle linkage arrangement, the poppet valve is not as adaptable as the linear spool valve when it comes to operating the handle in the same direction for reverse motor operation, also it is more difficult to pilot operate since some type of rotary actuator is required. Because of the need to connect inlet and exhaust passages to two chambers the valve body is relatively complex which further limits its adaptability to different motor requirements.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.